1. Field of the Invention
The invention relates to charged particle guns, and more particularly to field electron and ionization sources and ion guns for producing high current, medium energy ion beams and electrostatic lens systems for focusing the beams into small spot areas on a target.
2. Description of the Prior Art
There are numerous applications for ion guns, microprobes and the like capable of producing stable, high current, high resolution ion beams precisely focused onto very small "spot areas" of various targets. Gas phase ionization sources have been utilized, but are presently incapable of providing focused/high current beams of intermediate energy which are needed for certain applications. Liquid metal ionization sources have been experimentally used, but although they are capable of producing much higher currents than gas phase field ionization sources, liquid metal ionization sources have been highly unstable and unreliable due to variations and/or interruptions in their beam currents.
Accordingly, it is an object of the invention to provide a highly reliable liquid metal field ionization source.
Interest in production of submicrometer focused ion beams for various uses has led to a demand for ion guns capable of producing high beam currents and precisely focusing the beam into very small target spot areas. High current beams, especially ion beams, often have large energy spreads. Chromatic aberration of the electrostatic lens systems of ion or electron guns limits the minimum spot size obtainable with an ion or electron gun emitting beam having a high energy spread. Very bright field emission sources, such as thermal field electron emitters, gas phase field ionization emitters, and liquid phase field ionization omitters are capable of producing submicrometer electron or ion beams with tenths to hundreds of nanoamperes of current. However, ion beams produced by liquid metal field ionization sources have been found to have much greater beam energy spreads (.DELTA.E) than gas phase field ionization sources. The wide beam energy spread of beams produced by liquid metal field ionization sources limits the minimum spot size into which an ion beam can be focused by present two-element electrostatic optical lens systems for intermediate beam energies. In order to take advantage of the high level of brightness of such beam sources, it is necessary that their beams be focused by means of electrostatic lenses with low chromatic aberration in order to obtain submicrometer spots sizes. Up to now, no electron guns are capable of providing beam currents as high as 250 nanoampers in submicrometer spot for intermediate beam energies in the range from 2,000 to 30,000 electron volts. In situations typical of lithography or surface analysis, where high currents are desired in focused beam spots of approximately 1,000-2,000 Angstroms, the virtual source size has a relatively small effect on the final beam diameter. This is because the large acceptance angles in the electrostatic lens system necessary for high current result in discs of confusion due to chromatic and spherical aberration of the electrostatic lens system. Such discs of confusion are large compared to the virtual source size. In many cases the contribution of the beam diameter of the virtual source can be ignored and the current and beam spot size are determined by the electrostatic lens system alone.
Accordingly, it is another object of the invention to provide an optical system for focusing an ion beam or electron beam, which optical system is capable of focusing higher beam currents into smaller sized spots than has been previously possible with known field electron emitters, thermal field electron emitters, gas phase ion emitters or liquid metal ion emitters.
It is another object of the invention to provide an ion gun or electron gun capable of producing a large current and having an electrostatic lens system with a low chromatic aberration coefficient.
It is another object of the invention to provide an ion gun capable of focusing a high current intermediate energy beam with a relatively large energy spread onto a very small spot area.
It is another object of the invention to provide a liquid metal field ionization source in combination with an electrostatic optical system capable of focusing a high current beam onto a small spot area.
It is another object of the invention to provide a method of operating an ion gun having a liquid metal field ionization source to provide reliable, uniform beam currents and to minimize energy spread of the ion beam produced by the ion gun.
For many applications, a desirable characteristic of an ion gun is that it have the ability to produce a focused beam with continuous voltage variability while maintaining a fixed image and object distance.
A novelty search directed to the present invention uncovered the following references: U.S. Pat. Nos. 4,088,919; 3,911,311; 4,085,330; 2,809,314; 4,149,055; 4,146,810; 2,233,404; 3,143,680; 3,525,013; 3,551,728; 3,683,287; 3,793,550; 3,911,314; 3,949,230; 4,002,912; 4,032,810; 4,128,764 and 3,631,283 and also the article "A High-intensity Scanning Ion Probe With Submicrometer Spot Size" by Seliger et al, Applied Physics Letters 34(5), Mar. 1, 1979, page 310.
Accordingly, another object of the invention is to provide an ion gun capable of producing and focusing a high current ion beam on a very small spot area and also having the capability of varying the beam energy while maintaining a fixed image and object distance.